1. Field of the Invention
The invention concerns the field of integrated optics and fiber optic telecommunications and in particular filter-free wavelength converters for rejecting the optical input signal and guiding only the converted signal to the output of the converter.
Operation basis of wavelength conversion are cross gain modulation (XGM) or cross phase modulation (XPM) in semiconductor optical amplifiers (SOAs).
XGM is the method that is intensity-modulated a continuous wave (CW) light by gain-modulation with the intensity-modulated optical input signal in the SOA. As the intensity of the optical input signal increases, the gain is decreased and the intensity of the CW light also decreases. As the intensity of the optical input signal decreases, the gain increases and the intensity of the CW light also increases. Therefore, the information in the optical input signals can be transmitted to the required wavelength by using the CW light.
XPM method provides the phase modulation of the CW light with the optical input signal in the SOA. When the CW light propagates into the two paths, the path difference occurs due to refractive index change in the SOAs. The combination of two paths of propagated CW light cause the interference and the intensity of the CW light is modulated.
Wavelength converters can be operated in two ways in accordance with the input method of the optical input signal and the CW light.
First, the CW light and the optical input signal are launched in opposite directions each other. This method has an advantage that no filter is required to reject the optical input signal at the output of the converter. However, at both ends of the SOA, the recombination rate also decreases due to the carrier depletion, so that the conversion speed is degraded. Therefore, the waveform is seriously distorted and the conversion speed thereof decreases.
Second, the CW light and the optical input signal are coupled in the same direction for high-speed conversion. However, the filter is required in order to reject the optical input signal at the output of the converter or to output only the converted signal. This method provides conversion to the same wavelength.
Therefore, in the above-mentioned wavelength converter, it is necessary to use the CW light and the optical input signal in the same direction in order to improve the conversion speed. Thus, in order to simplify the configuration of the optical transmission system including wavelength converters, the function capable of separating or removing the optical input signal should be included.
2. Description of the Prior Art
Hereinafter, the embodiments of the configurations of wavelength converters will be explained with reference to the accompanying figures.
FIG. 1 shows a configuration of the multi-mode interference (MMI) mode converter. The MMI mode converter can separate a fundamental and a higher-order mode. (See U.S. Pat. No. 5,933,554, 1999. 8. 3) In this interferometer, because an output port is determined depending on the input port, the converted signal and the optical input signal can be separated. (See “Detailed experiment investigation of all active dual-order mode Mach-Zehnder wavelength converter”, Electron. Letters, Vol. 36, No. 15)
The operating principle of the MMI mode converter is as follows. The optical input signal 1 is injected into the input port 3 of a first MMI coupler 5 and it is propagated to the first SOA 7. The signal induces phase modulation of the CW light 2 and the signal 1 is absorbed in the ports 11, 12 of a second MMI coupler 9. The CW light is injected into the central input port 4 and it is divided into the first MMI coupler 5 and the third MMI coupler 6. The CW light 2 is phase-modulated in the first SOA 7 and amplified in the second SOA 8. Thereafter, it is divided into the second MMI coupler 9 and the fourth MMI coupler 10. Then, the two paths of light are combined in the output port 13 and only the converted signal is yielded by constructive or destructive interference.
FIG. 2 shows the conventional XGM method, and in this method, the SOAs 24, 25 are parallel connected to the input and output ports of the MMI couplers 23, 27. The optical output signal 29 and the converted signal 26 are separated. (See “Monolithic Integrated Parallel Amplifier Structure for Filter-Free Wavelength Conversion”, 13th IPRM, 2001 International Conference on Indium Phosphide and Related Materials, Conference Proceedings FB1-5). In this structure, the injection currents into both of the SOAs 24, 25 are same. XGM occurs between the optical input signal 22 and CW light 21 in the SOAs 24, 25. Therefore, wavelength conversion is performed. In cross state, the signal and CW lights are led to different output ports when they are coupled into the different input ports, which enables the spatial separation of the input signal and CW light, resulting in filter-free wavelength conversion.
The structure using the MMI mode converter of FIG. 1 can separate the converted signal 26 and optical output signal 29. However, the MMI coupler having multiple stages has a limitation that the employed structure is complicated. Also, there exists a problem that the performance of the wavelength converter depends on that of the MMI couplers. The structure of FIG. 2 can separate the converted signal and optical output signal. However, the MMI coupler and the SOA should be integrated so that the structure thereof is more complicated.